Use css-device-adapt constraining for legacy viewport tags.

Source/core/css/StyleRule.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * (C) 2002-2003 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2002, 2005, 2006, 2008, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 324 matching lines @@
 StyleRuleRegion::StyleRuleRegion(const StyleRuleRegion& o)
     : StyleRuleGroup(o)
     , m_selectorList(o.m_selectorList)
 {
     ASSERT(RuntimeEnabledFeatures::cssRegionsEnabled());
 }
 
 StyleRuleViewport::StyleRuleViewport()
     : StyleRuleBase(Viewport)
 {
-    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled());
 }
 
 StyleRuleViewport::StyleRuleViewport(const StyleRuleViewport& o)
     : StyleRuleBase(o)
     , m_properties(o.m_properties->mutableCopy())
 {
-    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled());
 }
 
 StyleRuleViewport::~StyleRuleViewport()
 {
 }
 
 MutableStylePropertySet* StyleRuleViewport::mutableProperties()
 {
     if (!m_properties->isMutable())
         m_properties = m_properties->mutableCopy();
@@ skipping 28 matching lines @@
         m_properties = m_properties->mutableCopy();
     return static_cast<MutableStylePropertySet*>(m_properties.get());
 }
 
 void StyleRuleFilter::setProperties(PassRefPtr<StylePropertySet> properties)
 {
     m_properties = properties;
 }
 
 } // namespace WebCore